My Thelma is also known as:
Tell-Ma - always spoken in baby voice, originated when she "ratted on" her older sister's naughty behavior
Thelma-girl
Sweet Potato
Pookie
Spooky Pook - a derivative of Pookie

My Weezie's real name is Louise. She is also known as:
Weeze
The Weezer
Weezie-girl

Not very creative, I know ...

I try to make up for it by making up new lyrics for showtunes, as in

Hello Thelma, well, hello Thelma!
You're sweet, you're so petite, you're Thelma-girl. (sung to the tune of Hello Dolly!)